1. Field of the Invention
The present invention relates to an input sound processor for determining the sound power at a specific point, more specifically, to an input sound processor for estimation of the power of a guide voice at a microphone.
2. Description of the Related Art
A typical navigation voice corrector for use in a navigation system changes the sound pressure level of a guide voice depending upon the ambient noise level to provide an intelligible guide voice even in noisy environments (see, for example, Japanese Unexamined Patent Application Publication No. 11-166835 (pages 3 to 6, FIGS. 1 to 10)). In this navigation voice corrector, a loudness-compensation-based gain determining unit corrects for the gain of a guide voice output from a loudspeaker based on the sound pressure levels of ambient noise and the guide voice at the position of a microphone, which is assumed to be a listening point of the guide voice. The sound pressure level of the ambient noise and the guide voice input to the loudness-compensation-based gain determining unit is represented by total sound power which is determined by summing powers at all of a plurality of frequency components.
However, the guide voice and the ambient noise actually reach the microphone at the same time, and it is not possible to extract only the guide voice from the sound collected by the microphone.
One typical technique for extracting a guide voice is estimation of the guide voice at the microphone based on the transfer characteristic from the loudspeaker to the microphone and the guide voice signal input to the loudspeaker. The total power of the guide voice at the microphone is determined by separately determining power at each frequency component of the guide voice and a square amplitude of the transfer characteristic at each frequency component and performing a product-sum operation at each frequency component (see, for example, Japanese Unexamined Patent Application Publication No. 2002-23790 (pages 3 to 4, FIGS. 1 to 2)).
The latter publication discloses that the power determined at each frequency component of an input voice is multiplied by the square amplitude of each tap coefficient indicating the transfer characteristic and a sum of the products is then calculated. It is therefore necessary to perform a product-sum operation at all frequency components, resulting in a large amount of processing. A high-performance processor is therefore required, which is costly.